Nucleoside inhibitors of viral polymerases and cancers are typically active as the nucleoside triphosphate (NTP). Conventional nucleoside drugs are phosphorylated by host cellular kinases to the triphopshate active form. The poor conversion of the nucleoside to NTP can often be attributed to the inability of nucleoside kinases to convert the nucleoside to the nucleoside 5′-monophosphate (NMP). NMP prodrugs have been used to bypass poor nucleoside kinase activity (Schultz, Bioorg. Med. Chem. 2003, 11, 885). Among these prodrugs, NMP phosphoramidates have been reported to increase intracellular concentrations of NTP compared to the nucleoside alone (McGuigan, J. Med. Chem. 1993, 36, 1048-1052). However, these NMP prodrugs are substrates for esterases and phosphodiesterases in the blood and other body tissues which can cleave the prodrug to a charged molecule or to the nucleoside, respectively. The charged molecule is then impermeable to the target organ or cell and the nucleoside is poorly phosphorylated intracellularly.
The development of a highly effective, non-toxic NMP prodrug is largely an unpredictable trial and error exercise requiring the balancing of the stability of the NMP prodrug in blood with the ability of the prodrug to reach a target organ or cell, be absorbed or actively taken up by the target cell, being efficiently cleaved to the NMP intracellularly and subsequently converted to a NTP that is selective for inhibiting the viral polymerase (Perrone, J. Med. Chem. 2007, 50, 1840-49; Gardelli, J. Med. Chem. 2009, 52, 5394-5407). For the case of an orally effective RdRp inhibitor for treating HCV infection, the NMP prodrug would need to be chemically stable to the conditions of the upper intestinal tract, be efficiently absorbed from the intestinal tract, survive the many esterases of the intestinal cells and blood, be efficiently extracted by the hepatocytes, and be cleaved to the NMP and subsequently converted to a NTP in hepatocytes that is specific for inhibiting the HCV NS5B polymerase.
The first generations of phosphoramidate prodrugs, such as AZT phosphoramidates (McGuigan 1993, J Med Chem 36 1048-1052) or BMS/Inhibitex INX-189 (WO2010/081082) were prepared as diasteroemeric mixtures at the phosphorous atom. Merck, Idenix and Achillion are believed to have clinical trials ongoing in relation to diverse phosphoramidate prodrugs of HCV-inhibitory nucleosides. Gemcitabine is an example of an anticancer nucleoside, which has been put into clinical trials by Nucana as a phosphoramidate prodrug (WO2005/01237). Nucana also works with phosphoramidate prodrugs of other anticancer nucleosides including clofarabine, fludarabin, cladribine (WO2006/100439) and FUDR (WO2012117246). Nucana's phosphoramidate prodrugs are typically a diastereomeric mixture at the phosphorous.
Subsequent generations of phosphoramidate prodrugs of an inhibitor of hepatitis C virus RNA-dependent RNA polymerase, such as Sofosbuvir (WO2008/121634) are diasteromerically pure at the phosphorous. Notably, the antiviral activity of phosphate prodrugs can markedly depend upon the chirality of the phosphorous in the prodrug (Gardelli, J. Med. Chem. 2009, 52, 5394-5407; Meppen, Abstracts of Papers, 236th ACS National Meeting, Philadelphia, Pa., United States, Aug. 17-21, 2008 (2008), MEDI-404.).
Pharmasset, later bought by Gilead, have described chiral phosphoramidate reagents in WO2010/135569, WO2011/123645 and WO2012/012465 useful in the preparation of sofosbuvir.
In view of the importance of anti-HCV therapeutics that are NMP prodrugs with chiral phosphorous atoms such as sofosbuvir or those described by Gardelli, et al., Perrone et al., and Meppen, et al., new efficient methods of producing chiral phosphates of these prodrugs are needed.
WO2012/012465 discloses methods and intermediates for preparing diasteromerically pure phosphoramidate prodrugs of nucleosides that are useful for the treatment of hepatitis C infection. The phosphorylating reagent disclosed therein is a phosphoramidate having an aryloxy or heteroaryloxy group, O—Ar, attached to the phosphorus atom as leaving group, i.e. a compound of formula Aa or Ab:
wherein Ar is a (C6-C20)aryl or heteroaryl which is substituted with one or more halogen, NO2, or (C1-C8)haloalkyl, and Ar is different from R4.